I think it's really an individual car and driver thing. I have experimented with a few cars where I have one reinforced and the other not and I don't find much difference in laps times. I do tend to find the non-reinforced cars feel better, more predictable and controllable on most corners. The reinforced cars just don't have the same feel and seem more prone to snap oversteer, given the same tune.
The last month or so, I've purchased probably 50 cars and haven't put it on any of them and it's working well.